1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for manufacturing a semiconductor device having a mushroom-shaped gate electrode, such as a Schottky gate field effect transistor or an integrated circuit including a Schottky gate field effect transistor.
2. Description of the Related Art
Gallium arsenide Schottky gate field effect transistors (hereinafter referred to as GaAs MESFETs) have excellent high frequency characteristics and can have high outputs. To realize MESFETs of high performance (high frequency, high output, high efficiency) and high reliability, various structures and manufacturing methods have been proposed.
To obtain high performance, for example reducing the gate resistance (R.sub.g), the source resistance (R.sub.s) and the source-drain capacitance (C.sub.gs) and increasing the transmitting conductance (g.sub.m) are important. To reduce the source-drain capacitance (C.sub.gs) and increase the transmitting conductance (g.sub.m), it is effective to reduce the gate length. To prevent increasing of the gate resistance (R.sub.g) resulting from reduction of the gate length, a mushroom-shaped gate electrode is effective.
A conventional method for manufacturing a GaAs MESFET having a mushroom-shaped gate electrode is disclosed in Japanese Patent Publication No. 4-72381.
FIGS. 2(a) to 2(d) are sectional views illustrating a conventional method for manufacturing a GaAs MESFET having a mushroom-shaped gate electrode.
First, an active layer 22 is formed by epitaxial growth or ion implantation in the upper part of a semiconductor substrate 21 made of semi-insulating GaAs. Then, a source electrode 23 and a drain electrode 24 are formed on the active layer 22, an insulating film 25 and a resist layer 26 which is a first mask layer are successively deposited. An opening 26a is formed in a part of the resist layer 26 corresponding to a region where a gate electrode is to be formed (FIG. 2(a)).
Next, an opening window 25a is formed in the insulating film 25 by selectively etching the insulating film 25 using the resist layer 26 having the opening 26a as a mask (FIG. 2 (b)).
The resist layer 26 is then removed, and another resist layer 27 which is a second mask layer is deposited. An opening 27a wider than the opening window 25a is formed in the resist layer 27. A recess 28 is then formed in the active layer 22 by etching the active layer 22 using the resist layer 27 and the insulating film 25 as a mask (FIG. 2(c)).
Finally, metal constituting a gate electrode 29 is formed on the semiconductor substrate 21 by a vapor deposition method, the resist layer 27 is removed and the metal film thereon is thereby also removed, and a GaAs MESFET 20 having a mushroom-shaped gate electrode 29 is obtained (FIG. 2(d)).
However, in this kind of manufacturing method, when wet etching with phosphoric acid or sulfuric acid is used to etch the insulating film 25, some slight nonuniformity of the amount of etching occurs over the substrate surface and among different lots. Thus, the intended width of the opening is not always obtained and uniformity is also sometimes poor. As a result there has been the problem that the device characteristics are nonuniform over the substrate surface and among different lots.
To make the amount of etching, i.e. the width of the opening window 25a, more uniform, dry etching such as reactive ion etching or ion milling can be used. However, in these cases there has been the problem that the surface of the active layer 22 is damaged and the device characteristics consequently deteriorate.
Also, this kind of manufacturing requires the steps of successively depositing the insulating film 25 and the resist layer 26 which is a first mask layer, forming the opening 26a in the resist layer 26, forming the opening window 25a in the insulating film 25 and then removing the resist layer 26, depositing the other resist layer 27 which is a second mask layer, and forming the opening 27a in the resist layer 27. Consequently there has been the problem that the process is complicated and cost is high.